Amplifier failure traced to burned out heater element in traveling wave tube (Hughes 774H). Tube failure associated with failure of grid pulse-upboard on amplifier mainframe (Card M0002). Problem traced to comparator on receiver end of optical fiber link that couples an amplified TTL signal to the grid driver. Comparator is a discontinued part; supplier found. Amplifier repaired. Installed a 15 year old tube as a temporary measure.